1. Field of the Invention
The present invention relates to an optical transmission system using an optical transmission line partly having a gain, in which a group velocity dispersion is not uniform with respect to a longitudinal direction.
2. Description of the Prior Art
When transmitting a high-speed, large-capacity optical signal, a deterioration of optical waveform due to a group velocity dispersion of a transmission optical fiber is a problem. To compensate for this waveform deterioration, it is effective to install a dispersion compensating fiber in a receiver, which fiber is designed to cancel the dispersion and dispersion slope of the transmission optical fiber.
A loss due to a dispersion compensating fiber is in general so large that cannot be neglected. To compensate for this loss, it is effective to install a lumped optical amplifier inside the receiver, or induce the distributed gain, such as a Raman gain to a dispersion compensating fiber using the optical pumping source which is installed in the receiver.
FIG. 6 shows the configuration of a prior art optical transmission system in which a dispersion compensating fiber is followed by an optical amplifier. In FIG. 6, numeral 1 indicates a transmitter, 2 is a receiver, 3 is an optical fiber forming an optical transmission line, 11 is an optical transmission circuit, 21 is a dispersion compensating fiber, 22 is an optical amplifier, and 23 is an optical receiver circuit.
Signal light received by the receiver 2, after being dispersion compensated by the dispersion compensating fiber 21, is amplified by the optical amplifier 22, and received by the optical receiver circuit 23. FIG. 6 also shows a power diagram, showing that the input power to the optical amplifier 22 is considerably reduced due to a loss of the dispersion compensating fiber 21.
FIG. 7 shows the configuration of a prior art optical transmission system installed with an optical amplifier in front of the dispersion compensating fiber. In FIG. 7, numeral 1 indicates a transmitter, 2 is a receiver, 3 is an optical fiber forming an optical transmission line, 11 is an optical transmission circuit, 22 is an optical amplifier, 21 is a dispersion compensating fiber, and 23 is an optical receiver circuit.
Signal light received by the receiver 2 is amplified by the optical amplifier 22, dispersion compensated by the dispersion compensating fiber 21, and received by the optical receiver circuit 23. FIG. 7 also shows a power diagram, in which the input power to the dispersion compensating fiber 21 is increased to more than a certain level by the optical amplifier 22, so as to compensate for the loss of the dispersion compensating fiber 21 and prevent S/N deterioration in the optical receiver circuit 23 (K. Hagimoto et al., OAA'90, Technical Digest, TUA2, 1990).
FIG. 8 shows the configuration of a prior art optical transmission system in which Raman gain is induced in the dispersion compensating fiber. In FIG. 8, numeral 1 indicates a transmitter, 2 is a receiver, 3 is an optical fiber forming an optical transmission line, 11 is an optical transmission circuit, 21 is a dispersion compensating fiber, 23 is an optical receiver circuit, 24 is a optical pumping source, 25 is a pumping light coupler, and 26 is an isolator.
Signal light received by the receiver 2 is dispersion compensated by the dispersion compensating fiber 21 and received by the optical receiver circuit 23. The dispersion compensating fiber 21 is backward pumped using pumping light coupler 25, and blocked by the isolator 26. FIG. 8 also shows a power diagram, showing that loss and Raman gain in the dispersion compensating fiber 21 are balanced and the optical power is maintained at a constant value (P. B. Hansen., Elec. lett., 34, pp1136-1137, 1998).
However, as shown in FIG. 6 and FIG. 7, in the configuration where the optical amplifier 22 is disposed in the receiver 2, the signal quality is deteriorated for the reason described below. That is, when the optical amplifier 22 is placed after the dispersion compensating fiber 21 (FIG. 6), due to a loss by the dispersion compensating fiber 21 in addition to a loss L of the optical fiber 3, SIN of the main signal light is degraded at the output of the optical amplifier 22, resulting in a degradation of the sensitivity of the optical receiver circuit 23.
When the optical amplifier 22 is placed in front of the dispersion compensating fiber 21 (FIG. 7), since the dispersion compensating fiber 21 is smaller in core diameter than an ordinary fiber, it is necessary to limit the output level of the optical amplifier 22 to a level at which a nonlinear optical effect induced in the dispersion compensating fiber 21 can be neglected, thus limiting a loss compensation range of the dispersion compensating fiber 21.
On the other hand, when Raman gain is induced in the dispersion compensating fiber 21 (FIG. 8), waveform degradation of the main signal caused by a nonlinear optical effect, which restrict the system performance of the configuration shown in FIG. 7, can be relaxed. However, S/N of main signal light received by the optical receiver circuit 23 is determined by the loss L of the optical fiber transmission line 3, which cannot be improved any further. To utilize a Raman gain, the dispersion compensating fiber 21 is required to have a length of several tens of km, which increases propagation delay inside the receiver 2, resulting in an increased delay in the optical transmission system.